1. Field of the Invention
This invention relates generally to organic polymeric thin films for photonic applications. Specifically, the invention focuses on the process for making second-order nonlinear optical polymeric (hereinafter, NLOP) films.
2. Description of the Prior Art
The following information is provided as a brief overview of technology relevant to this invention. For a more detailed discussion involving this technology please refer to U.S. Pat. No. 5,247,055 issued Sep. 21, 1993 to Stenger-Smith et al., U.S. Pat. No. 5,520,968, issued May 28, 1996 to Wynne et al., and the book, Polymers for Second-Order Nonlinear Optics, G. A. Lindsay and K. D. Singer, Eds., Am. Chem. Soc. Advances in Chemistry Series 601, Washington, D.C., 1995.
Organic polymeric thin films for photonic applications has been a rapidly evolving area of research for over ten years. One class of materials within this field, NLOP films, has potential for breakthroughs in low cost integrated devices for the telecommunication and data-communication industries. Key components of this new technology are electro-optic (EO) waveguides made from second-order nonlinear optical polymer films. These waveguides have the potential to switch optical signals from one path to another and also to modulate the phase or amplitude of an optical signal.
The molecular origin of optical nonlinearity derives from the electrical polarization of the chromophore as it interacts with electromagnetic radiation. The molecular structure of the chromophore and its orientation govern the nonlinear optical properties of the system. Furthermore, it is the polymer structure that dictates the processability and temporal stability of the final product.
In order for films to have a large NLO coefficient, they must contain a high concentration of asymmetrical, highly polarizable chromophores arranged in a highly polarized configuration. In the past few years, several types of polymers have been developed which are effective in EO modulation of optical signals.
As previously noted, the polymer structure dictates the processability and temporal stability of the final product. Chromophores in the polymers are usually aligned by electric field poling near the glass transition temperature. When the film is cooled, alignment of the chromophores is frozen in the desired position.
Guest-host systems:
This class (guest-host systems) of nonlinear optical polymers consists of small molecular, asymmetric chromophores dissolved in glassy polymers. These solid solutions typically contain small molecular chromophores dissolved in high molecular weight polymers. Guest-host systems typically contain 10-20% by weight of the chromophore. A drawback of guest-host materials is the fact that the glass transition temperature of the polymer decreases due to plasticization by the chromophore. In addition, chromophores in guest-host systems are to some degree labile (they diffuse to the surface of the film and may evaporate at elevated temperature). Chromophores on the surface of guest-host films can be absorbed through the skin. These chromophores are often toxic, mutagenic, teratogenic and carcinogenic. By attaching these chromophores to a high molecular weight polymer, which cannot be absorbed through the skin, the health hazards are greatly minimized.
In spite of these limitations, guest-host systems may have practical applications if the polymer selected has a high glass transition temperature (&gt;250.degree. C.) and a large chromophore is utilized.
Sidechain Polymers:
Sidechain polymers consist of asymmetric chromophores chemically attached at one point pendant to the backbone of the polymer. For example, the attachment occurs at the electron accepting end or at the electron donating end of the chromophore. Compared to guest-host systems, sidechain polymers have a much greater temporal stability at a given glass transition temperature and for a given chromophore.
Mainchain Polymers:
In mainchain polymers the chromophores are chemically attached (linked) at both ends resulting in the majority of the chromophore forming part of the backbone. The unique characteristic of this class of polymers is that the asymmetric chromophores can be linked in a head-to-tail pattern (isoregic), head-to-head pattern (syndioregic), or in a random head-to-head and head-to-tail (aregic) pattern. Because chromophores in mainchain polymers are linked at both ends in one of these three types of patterns, the chromophores have one less degree of freedom of motion relative to sidechain polymers. Therefore, mainchain chromophoric topology is, in principle, more stable than sidechain chromophoric topology.
Polar Order:
Second-order nonlinear optical properties require that the chromophore orientation in the film is noncentrosymmetric. Two primary techniques used to impart polar order in the film are elevated temperature electric-field poling and room temperature Langmuir-Blodgett processing. Recently, photopoling has been utilized for imparting polar order at room temperature for azo-containing chromophores.
Stability:
There are a number of different types of stability relevant to asymmetrical chromophores. Physical stability refers to the stability of the polar chromophore alignment to relaxation into a nonpolar state. Chemical stability refers to the integrity of the chemical structure of the chromophore, for example, against oxidation or hydrolysis. Photochemical stability refers to the stability of the chromophore to irradiation by light, especially in the presence of oxygen and water. Temporal stability refers to how well the physical, photochemical and chemical stability are maintained at a given temperature. Finally, processing stability refers to how well the polymer handles film processing procedures and various packaging operations. All of the above types of stability are critical if long term optical stability is to be achieved.
Electric-Field Poling:
Thin polymer films are prepared for poling by spin-coating a liquid solution of the polymer (about a 10 to 30% concentration) onto a solid substrate. The solvent is removed by baking the film just above the glass transition temperature (Tg). After baking, an electric field is applied across the film in one of two ways:
1) By corona poling the film on a grounded conductor plane near the film's Tg for 1 to 150 minutes. PA1 2) By charging two electrodes contacting the film heated to Tg for 1 to 150 minutes. Either of these processes can create an electric field of fifty to several hundred volts/micron across the film. The film is then cooled with the field on. After the external field is removed, a net alignment of dipole moments can remain locked in the film for long periods of time, providing that the temperature of the film remains well below any solid state transition, such as the Tg. PA1 1) Lowering monolayer viscosity by use of higher subphase temperatures, choice of subphase ions, or change of pH. [Please refer to the following for more information on this subject, "Insoluble Monolayers at Liquid-Gas Interfaces" G. L. Gaines, Interscience Publishers, New York, 1966.] PA1 2) Utilizing alternative monolayer compression schemes such as the flowing subphase [Please refer to the following for more information on this subject, Advanced Materials 1991, 3(1), 25-31]. PA1 1) It eliminates the entire electric-field poling step. PA1 2) It eliminates the dilution effect of the hydrophobic alkyl groups. PA1 3) It creates stronger ionic bonds between the polymer chains. PA1 4) It can double the rate of making films of a given thickness and of a given NLO coefficient compared to prior LB art.
There are several problems associated with electric-field poling. First, the polymer utilized must be heated to high temperatures. At these high temperatures thermal disordering of the chromophores works against the torque of the electric field resulting in the chromophores being less well ordered. In addition, polymers containing formal charges are very difficult to pole with an electric field because the charges tend to migrate through the polymer causing dielectric breakdown (i.e. shorting out the electrode).
Langmuir-Blodgett (LB) Processing:
In conventional LB processing, the polymer molecules are designed to have hydrophilic and hydrophobic groups which cause the polymer to float on the gas-liquid interface in a preferred confirmation. These hydrophilic/hydrophobic forces are useful in removing the centrosymmetry by orienting the chromophores normal to the plane of the film.
To make films by LB processing, an organic compound is floated on a liquid, eg. water, ethylene glycol or other aqueous solutions, in a trough. A solid substrate is dipped through the gas-liquid interface depositing a single molecular layer on the substrate. Thicker films comprised of multilayers of polymers are built up by repeatedly dipping the substrate into and/or out of the trough, depositing one layer per stroke.
One of the main advantages that conventional LB processing has over electric-field poling is that LB processing may be carried out at room temperature (or lower). Furthermore, unlike electric field poling, formal ionic charges on the polymer need not hinder the ordering process.
Previous materials utilizing the LB methodology for the fabrication of waveguides (U.S. Pat. No. 5,162,453 issued Nov. 10, 1992 to Hall et al., U.S. Pat. No. 5,225,285 issued Jul. 6, 1993 to Hall et al., U.S. Pat. No. 4,830,952 issued May 16, 1989 to Penner et al, and U.S. Pat. No. 4,792,208 issued Dec. 20, 1988 to Ulman et al.) have suffered from thermal instability due to the presence of low melting alkyl and fluoroalkyl hydrophobic chains. One strategy to increase the thermal stability of LB films is the use of interlayer and/or intralayer covalent bonding (i.e. crosslinking). Another strategy is to attach chromophores to rigid polymer backbones. However, attaching sidechain chromophores to polyimides [Please refer to the following articles for more information on this subject: Thin Solid Films, 244 (1994) 754-757, and Langmuir, 10 (1994) 1160-1163] failed to provide stable multilayer NLOP films.
A limitation of LB technology is the amount of time required to build up films of sufficient thickness (&gt;0.5 micrometers) for waveguiding. Two ways that the rate of deposition can be increased on the substrate without sacrificing film quality are:
As mentioned earlier, the classical LB processing technique requires that the material self-assemble into noncentrosymmetric order at an interface between gas and aqueous liquid through a balance of hydrophobicity and hydrophilicity. Typically, by design, functional groups are introduced into the polymer chemical structure to bring about preferential chromophore orientation. These functional groups, especially the alkyl groups which are used for hydrophobicity, lead to a lowering of the Tg and a dilution of the concentration of chromophores. Dilution causes a lowering of the nonlinear optical coefficient of the waveguide.
This invention is unique for a number of reasons: